1. Field of the Invention
The present invention relates to a starter with a planetary gear reduction facilities which is adapted to decrease an impact force that develops when an internal combustion engine is started.
2. Description of the Prior Art
FIG. 1 is a side view, partially in cross-section, showing a conventional starter with a planetary gear reduction facilities. In FIG. 1, a D.C. motor 1 for starting an internal combustion engine (not shown) includes a cylindrical yoke 2 which forms a magnetic path, and a ferrite permanent magnet member magnetic pole 3 which is fixed to an inner peripheral surface of the yoke 2. The magnetic pole 3 and the yoke 2 form a magnetic field means. The D.C. motor 1 also includes a flange 4 which closes an opened end of the yoke 2, and a rotary shaft 5 is rotatably supported by the flange 4 via a bearing 6. An armature 7 is mounted to the rotary shaft 5, and an armature winding 8 is applied to the rotary shaft which completes the construction of the D.C. motor 1.
In addition to the D.C. motor, the conventional starter contains a spur gear 9 which is fitted to a projected portion 501 of the rotary shaft 5, and the projected portion 501 extends outwards from the flange 4. A cylindrical gear 11, which has a gear on its inner peripheral surface, is rotatably mounted on an inner peripheral surface of the yoke 2 so that it is opposite the spur gear 9. Two planetary gears 12 engage the ring gear 11 and the spur gear 9, and these gears transfer torque between the ring gear and the spur gear. An output rotary shaft 13 is rotatably supported at the projected portion 501 of the rotary shaft 5 via a bearing 13', and it is provided with a collar-like portion 131. Pins 14 are fittedly inserted into holes 132 which are formed in the collar-like portion 131. The pins 14 rotatably support the planetary gears 12 via bearings 15. A flange 16 is in close contact with an end surface 111 of the ring gear 11, as well as with the yoke 2, so that the ring gear 11 is clasped between the flange 4 and the flange 16. In addition, the flange 16 is in contact with the outer surface of the collar 131 of the output rotary shaft 13 to prevent the output rotary shaft 13 from sliding to the right in FIG. 1. A front bracket 17 fixedly secures the yoke 2 and the flange 16.
The starter shown in FIG. 1 operates as follows: If an electric current is supplied to the armature winding 8, and a magnetic field is generated by the yoke 2 and the magnetic pole 3, a rotary power is generated which causes the rotary shaft 5 to rotate. Accordingly, the planetary gears 12 are rotated due to their engagement with the spur gear 9, and they undergo a planetary motion while simultaneously engaging the ring gear 11. Thus, the speed of rotation of the rotary shaft 5 is decelerated and transmitted to the output rotary shaft 13. An overrunning clutch (not shown) is spline-coupled to the output rotary shaft 13, and an internal combustion engine (not shown) is started by a rotation power of a pinion (not shown) which unidirectionally engages the overrunning clutch.
In use, the internal combustion engine (not shown) is stationary at the initial moment when it is engaged by the pinion. Hence, a lock torque is instantaneously generated in the D.C. motor at the time of the above-mentioned engagement. Thus, an impact load is generated in the power system which includes the D.C. motor and the various gears. Often, however, the planetary gear 12 does not have sufficient strength to withstand this impact load, and, accordingly, its reliability is poor.